Feedback amplifier



March 19, 1935.

VG. w. BARNES- FEEDBACK AMPLIFIER Filed Aug. 26, 1952 3 Sheets-Sheet 11mm mas mrms'maz 7 COUPL/N NETWORK cou u/va NETWORK B 7 a 1E A) A' 1 i256%2795 FIG H/PE 6 f 5 If PHASE CONTROL 4 NETWORK F762 F I63 REJ? HPF AI a f s f 5 no.4 FIG-.5

I HPF T HPF f s a f 5 FIG? HPF 5 J a 6 f s f INVENTOR G. (BARNES ATTONE) March 19, 1935; G. w. BARNES 1,994,457

FEEDBACK AMPLIFIER Filed Aug. 26, 1932 3 Sheets-Sheet 2 INVENTOR G. W BARNES BV A TTORNEY March 19, 1935. v 5 w BARNES 1,994,457

FEEDBACK AMPLIFIER Filed Aug. 26, 1932 3 Sheets-Sheet 3 FIG. 9

PHASE-0F mm INTERNAL A flab REGENERATION F FINAL PHASE OF Zlfl WITH ALOW PASS FILTER (E) 200 I80 PHASE 0F =cHA/vaE ab ab l IN PHASE OF UflI201 Q0", so; PHASE oF ap WITHOUT INTERNAL REGENERATION -40| 3. IKILOCVCLES- PEI? o a 2 a 4 se7a9uo' 2 a 4 56 00: Q PHASE 01-75 0 40 IPHq E oFA LOW PASS FILTE/i h f} =40o/ cc. 8009: so;

A TTORNEY Patented Mar. 19, 1935 UNITED STATES FEEDBACK AMPLIFIER GeorgeW. Barnes, East Orange, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationAugust 26,

21 Claims.

This invention relates to wave translating systems, as for example,electric wave amplifiers.

An object of the invention is to control feedback or-retroaction in suchsystems.

In one specific aspect the invention is a multistage vacuum tubeamplifier system of the general type in which waves are so fed back fromthe output to the input as to reduce the gain of the amplifier, in orderto reduce unwanted modulation or non-linear effects and render the gainstability greater than it would be without feedback. That type ofamplifier is disclosed for example in the copending application of H. S.Black, Serial No. 606,871, filed April 22, 1932 for wave translationsystems, assigned to the assignee of this application, and in BritishPatent No. 317,005.

In such amplifiers, where tube modulation reduction for modulationcomponents of givenfrcquencies is to be large it is proportional to. thegain (for those modulation components) in a single trip around theclosed feedback loop and consequently that gain should be large. Themodulation components that it is desired to reduce by feedback areusually waves of frequencies within the utilized frequency range, e. g.within the range of the frequencies of the signal waves to be amplifiedby the amplifier. In practice, when the loop gain (i. e., the decibelgain for a single trip around the loop) is large for the frequenciesof'the utilized frequency range, it is greater than zero for some higherfrequency and if the loop phase shift (i. e. the phase shift experiencedby waves in passing once around the loop) is zero or a multiple of 360for any frequency at which the loop gain exceeds zero decibels theamplifier may sing at that frequency. Moreover, passive networksintroduced in the loop to contribute a component of loop attenuationincreasing with frequency ordinarily introduce a component of loop phaseshift that tends to lower the frequency, and raise the gain, at whichthe loop phase shift reaches a given multiple of 360. To avoid thesinging condition, it is desirable to control the loop phase shift andthe loop gain carefully with respect to the entire frequency spectrum.(Such control is desirable also for other reasons, as for example toprevent loop V phase shift from causing increase in the gain of theamplifier, which increase may be undesirable because accompanied by acorresponding increase in the modulation products. This increase in gainmay become limiting factor determining the permissible loop phase shiftat high frequencies where it becomes difficult to obtain suilinot alimiting factor. For example, assuming the amplifier does not sing, theamplifier gain change produced by feedback will be within 1 decibel ofthe gain around the feedback loop regardless cient feedback. At otherfrequencies it is usuallyof phase shift if the gain around the loopexceeds 1932, Serial No. 630,553

(C1. ES -44) 20 decibels. .Further, the feedback does not increase theamplifier gain if the loop gain exceeds 6 decibels). In particular, inorder to avoid danger of singing it is desirable that the variation ofloop phase shift with frequency, over the frequency range for I whichthe loop gain exceeds zero decibels, be maintained less than 360. If thevalue of the loop phase shift were maintained at '-180 it would be asremote as possible from the potential singing values of and multiples of360.

In designing an amplifier with negative feedback for distortionreduction, assuming the vacuum tube or tubes of each stage in the loopto introduce a phase shift having a constant componentof 180 (inaddition to any component due to interelectrode capacitance, forexample), the number of vacuum tube stages used in the loop may be madeeither odd or even, to facilitate control of singing tendency. Thequestion whether an odd or an even number is more suitable will dependupon whether the loop is made to have phase reversing means other thanthe tubes, (as for example, effective reversal or crossing over of ,theconnections somewhere in the loop in the manner disclosed in thecopending application ofE; Peterson, Serial No. 617,557, filed June 16,1932, (Patent No. 1,955,827, April 24, 1934), for Wave translatingsystems, assigned to the assignee of this application), and upon whatother phase shifts are present in the loop. If, over the frequency rangefor which the loop gain exceeds zero decibels (herein called thefrequency range of loop 'gain); the constant component of the total loopphase shift is any odd multiple of 180, then it is necessary that thetotal variation of the quency range be maintained withinlimits of.

+180 and -180 in order to make the total loop phase shift differ fromzero and every multiple of 360 for every frequency in that frequencyrange.

The dimculty of insuring that the variation of loop phase shift withfrequency is maintained less than 360 or within the required limits overthe frequency range of loop gain is in general increased by the factthat, (as brought out in the above'mentioned copending application of H.S. Black), when the distortion reduction and associated amplifier gainreduction produced by feedback action is to be large the gain of theamplifier without feedback must then correspondingly exceed the gainrequired with feedback; because when the gain without feedback, requiredto produce the desired amount of distortion reducing feedback and thedesired amount of gain with feedback, necessitates use of a plurality ofstages and a plurality of interstage coupling circuits the phase shiftsaround the closed loop may become large. For example, they may becomelarge at frequencies well above the utilized range because of shuntcapacitance, for instance, tube and wiring capacities. The singingtendency may become particularly troublesome when the amplifier iscalled upon to transmit wide frequency bands extending to very highfrequencies, for example.

Moreover, as brought out in the above mentioned copending application ofH. S. Black, it is often highly desirable to connect an attenuationequalizing network (having its attenuation increasing with frequencyover the utilized frequency range) in the feedback path of the amplifier, for equalizing line attenuation; and the presence of thisequalizer may greatly increase the difiiculty of insuring that thevariation of loop phase shift with frequency is maintained within therequired limits over the frequency range of loop gain.

These difficulties caused by (relations of attenuation to) phase shiftin tube and wiring capacities, attenuation equalizing networks in thefeedback path, and othercircuit elements arise from the fact that suchpassive networks have the slopes of their phase-frequencycharacteristics all of the same sign, conventionally called positive, orthe characteristics of the networks that give positive phase shift trendoppositely to those of the networks that give negative phase shift, withthe result that the passive networks yield a total loop phase shift thatvaries widely with frequency, becoming high at high frequencies in thefrequency range of loop gain, the variation tending to exceed themaximum permissible 360 or :180 for that frequency range. That is, withboth the passive networks that produce positive phase shift and thepassage networks that produce negative phase shift having the slope oftheir phase-frequency characteristics positive (the positive phaseshifts increasing with frequency increase and the -negative phase shiftsand (2).

1 decreasing with frequency increase), it results that at highfrequencies in the frequency range of loop gain the total of thenegative phase shifts in the loop is too small to counteract the totalof the positive phase shifts sufficiently to prevent the loop phaseshift from becoming zero or a multiple of 360 or crossing the zero axisat a high frequency in the frequency range of loop gain, (much less toyield, over the frequency range of loop gain, the 180 loop phase shiftwhose value is as remote from the potential singing values as itisgpfissible to have the value of the loop phase shift). This difficultycould be overcome if suitable corrective networks were availablehaving 1) attenuation increasing with frequency, without positive slopeof phase shift-frequency characteristic, or (2) negative slope of phaseshiftfrequency characteristic without attenuation decreasing withfrequency increase, or (3) both (1) Unfortunately, (a) passive networksthat have attenuation increasing with frequency not only do not havenegative slope of phase shift-frequency characteristic, but havepositive slope of that characteristic; (1)) passive attenu- V atingnetworks that give negative slope of phase shift-frequencycharacteristic do so only over a limited frequency range, and at highfrequencies the attenuation decreases with frequency; and (c)(non-dissipative networksnot only do not have negative slope of phaseshift-frequency characteristic but have positive slope of thatcharacteristic. However, as explained in the above mentioned copendingapplication of H. S. Black, by inserting in the feedback path of anamplifier with negative or gain reducing feedback and loop gain largecompared to unity a passive network whose (negative) phase shiftincreases with frequency (i. e. whose phase shift-frequencycharacteristic has a positive slope), the amplifier phase shift can bemade to be substantially the negative reciprocal of the phase shift ofthe passive network, and thus to compensate for or counteract thevariation of phase shift with frequency represented by the positiveslope of the phase shift-frequency characteristic of a circuit in whichthe amplifier is connected or of any apparatus associated with thecircuit, so that, for example, by making the phase shift-frequencycharacteristic of the passive network substantially identical with thatof the circuit or apparatus, the variation of the phase shift ofthecircuit or apparatus with frequency can be completely compensated foror annulled if desired. As further pointed out in that copendingapplication, a feedback amplifier can be used as part of the forwardlytransmitting portion of a multiple feedback amplifier to alter the phaseshift with a view to making the latter amplifier more readily complywith the requirements for freedom from singing.

The present invention, in its specific aspect mentioned above, is anegative feedback amplifier system in which a portion of the forwardlytransmitting part of the closed feedback loop, which portion includes apart only of the tubes of the forwardly transmitting part of the closedfeedback loop, is itself a negative feedback amplifier, for altering thephase shift around the outer closed feedback loop to reduce singingtendency. The feedback path of the local or internal feedback loopcomprises frequency selective phase shift correcting or phase shiftcompensating means which includes a high pass filter with cutofffrequency at or above the utilized frequency range. The high pass filterhas a positively sloping phase shift-frequency characteristic thatcauses the internal loop to produce in the external loop a phase shift,that has a negatively sloping phase shift-frequency characteristic,combining with or adding to the phase shift (that has a positivelysloping phase shift-frequency characteristic) of the remainder of theouter loop (i. e. the part of the outer loop that is not common to theinner loop) to increase the minimum departure of the loop phase shift(of the outer loop) from 0 and multiples of 360 over the frequency rangeof loop gain for the outer loop. Or the internal loop produces in aportion of the external loop a phase shift combining with that of theremainder of the loop to make the phase shift of the (total) outer loopdiffer from zero and every multiple of 360 at every frequency for whichthere is an (outer) loop gain, or prevent the phase shift of the (total)outer loop from becoming zero or any multiple of 360 at any frequencyfor which the loop gain around the outer loop exceeds zero.

The frequency selective phase shift correcting or compensating meansintroduces in the outer loop, principally by the high pass filter,frequencyselective loss (or gain reduction) which tends to lower theouter loop gain at which the outer loop phase shift reaches 0 or amultipleof 360; and the phase shift (including that of the high passfilter) which the frequency selective phase shift corrective meansintroduces in the outer loop is such that it does not tend to make theouter loop phase shift reach zero or a multiple of 360 at a loweredfrequency and attendant higher outer loop gain, i. e., does not tend tolower the frequency. at which the loop phase shift reaches zero or amultiple of 360, but on the other hand is such that it tends to increasethe frequency at which the loop phase shift reaches zero or a multipleof 360. (The effect upon the outer loop gain of variation of the loopphase shift for the inner loop is small; because-the effect upon thegain of the inner amplifier is small since, as brought out in the abovementioned copending application of H. S. Black, in a feedback amplifierwith loop gain large, only slight change is produced in the gain of theamplifier by varying loop phase shift of the closed feedback loop thatincludes the feedback path from the output to the input of theamplifier.)

Since the high pass filter has its cut-off frequency at or above theutilized frequency range it. suppresses transmission through the filterof waves of frequencies of the utilized frequency range, so that forwaves of those frequencies there is substantially no feedback in theinternal loop and the amount of the local negative feedback and theconsequent reduction of (distortion reducing) gain around the outer loopis zero or much smaller than for waves of higher frequencies, whichordinarily have greater tendency to produce singing because of thelarger phase shifts that they experience in traversing the outer closedfeedback loop. That is, the high pass filter causes the reduction thatfeedback produces in the gain of the inner feedback amplifier to berelatively small for utilized frequencies (where reduction of this gainwould decrease the distortion reduction produced by the feedback aroundthe outer loop, for a given over-all gain of the amplifying system), andto be relatively great for higher frequencies (there tending to cut offthe gain around the outer loop and so tending to prevent singing aroundthe outer loop at those frequencies regardless of the phase shift aroundthat loop). Thus, with the high pass filter included in the phasecompensating means, the internal feedback is such as to not onlyintroduce in the external feedback loop. a phase shift that is helpfulin preventing singing, but to also reduce singing tendency by reducingthe gain around the outer loop above the utilized frequency range,without materially reducing that gain in the utilized frequency rangewhere it is useful in reducing distortion.

If the phase shift of the phase corrective network (i. e. of thefeedback path of the internal loop) were made to be substantially thesame as that of the portion of the external loop that is not common tothe internal loop, the phase shift introduced in the external loop bythe internal loop would be substantially the negative reciprocal of thisphase shift, and then the phase shiftfrequency characteristic of theexternal loop would be completely compensated for in the sense that itwould be stabilized at 180", i. e. the phase shift around the externalloop would then be 180 and would not vary with frequency. In practicethis condition need be approached only to the extent of preventing thephase shift around the outer loop from becoming zero or a multiple of.

360 in the frequency range of loop gain. That is, the extent of thephase compensation or phase correct-ion need be only this, that forevery' frequency of the frequency range of loop gain for the outer loop,the sum of the negative reciprocal of the phase shift of the phasecompensating or phase correcting network (i. e. of the negativereciprocal of the phase shift of thelocal feedback path of the innerloop) and the phase shift of the portion of the outer loop that is notcommon to the inner loop, shall not be substantially zero or anymultiple of 360.

, The above mentioned specific aspect of the invention is, then, amultiple negative feedback amplifier, or amplifier with multiplenegative feedback connections providing a plurality of distinct closedfeedback loops not reducible to merely a single closed feedback loop,one of the closed feedback loops being located in the forwardlytransmitting portion of another of the closed feedback loops, and theone loop having in its feedback path frequency selective phasecorrective means including a high pass filter with cut off at or abovethe upper limit of the utilized frequency range, for contributing to theouter'loop a phase shift that has a negative phase shift-frequencycharacteristic for reducing the singing tendency of the outer loop,while at the same time contributing to the outer loop a gain variationwith frequency that reduces the singing tendency of the outer loopwithout materially decreasing the distortion reduction resulting fromfeedback around the outer loop at frequencies in the utilized frequencyrange.

In the case of the internal closed feedback loop, as in the case of theexternal loop, in order to avoid singing the phase shift around the loopshould not be allowed to become zero or any multiple of 360 at anyfrequency at which there is a gain around the loop; but this requirementordinarily is less difficult to meet in the case of the internal loop,where fewer amplifier stages are required, and where no attenuationequalizer is required, and where the high pass filter reduces thefrequency range of loop gain, and where the required modulationreduction (which, if large in the case of the outer loop, requires largegain around the outer loop) is not large.

However, the phase shift-frequency characteristic of the inner feedbackpath'should be chosen to meet this requirement for avoiding danger ofsinging around the inner loop, as well as to meet the similarrequirement for avoiding danger of singing around the outer loop; andfor this reason it is often desirable to have the inner feedback pathinclude means for producing a phase reversal, or other desired phaseshift, over the frequency range of inner loop gain. For example, if thetube or tubesof the inner loop gave-a 180 phase shift and the high passfilter introduced a 180 phase shift at a frequency in its pass band nearthe lower edge of .the band, and if these were the only phase shifts inthe inner loop at that frequency, and if that frequency were a frequencyof inner loop gain, then there would be danger of the inner loopsinging; and this singing danger could be avoided by the means forproducing a phase reversal, or other appropriate phase shift, justmentioned. Often, also, the phase shifting means is desirable tofacilitate meeting the requirements as to the phase shift around theouter loop over the frequency range of outer loop gain. .The phaseshifting means may be, for example, a transformer, all pass structure ora special network or some combination of any or all.

While the high pass filter should be as effective as possible in theutilized frequency range (and produce as little adverse effect aspossible in the quency of the high pass filter, the high pass filter. orthe local feedback path does not give suificient negative feedback toprevent the outer loop gain from exceeding zero. vention, a low passfilter having its cut-off below such high frequency but above thecut-01f frequency of the high pass filter may be connected in the outerloop, preferably in the outer feedback path. A further advantage of thelow pass filter in the outer feedback path is that it causes theamplifier to suppress frequencies above the cutoff frequency of the lowpass filter, since, as

brought out in the above mentioned copending application of H. S. Black,a low pass filter connected in the feedback path from the output to theinput of a negative feedback amplifier acts as a high pass filter in themain transmission circuit.

As indicated by the foregoing discussion, loop phase shift and loop gainpresent important limitations in operation of feedback amplifiers,especially the loop phase shift and the loop gain at high frequencies inoperation of wide band negative feedback amplifiers, for reducingdistortion by feedback action and more especially where transmissioncontrol means producing phase shift, as for example attenuationequalizing means, are included in the feedback means.

Objects of the invention are to control such loop phase shift or loopgain, or both.

It is also an object of the invention to reduce singing tendency and/ orto increase the distortion suppression obtainable in such amplifiers.

Other objects and aspects of the invention will be apparent from thefollowing description and claims.

In the accompanying drawings,

Fig. 1 is a circuit diagram of an amplifier embodying a form of theinvention;

Figs. 2 to 7 are fragmentary diagrams showing various modifications ofparts of the amplifier circuit of Fig. -1;

Fig. 8 is a circuit diagram of an amplifier embodying another form ofthe invention; and

Fig. 9 shows curves for facilitating explanation of the operation of theamplifier of Fig. 8.

In Fig. 1 a negative feedback amplifier A amplifies waves received byinput transformer T from line or circuit L and transmits the amplifiedwaves through output transformer T to line or circuit L. The amplifier Acomprises a forwardly transmitting path P including vacuum tubes 1, 2and 3, an outer feedback path F including an attenuation equalizer 4 forequalizing attenuation of line L, and an inner feedback path 1 includinga filter 5 and a phase shift adjusting network 6. The filter 5 may be ahigh pass filter or a network having somewhat similar character--istics. The path F is from the output of amplifier A to the input of theamplifier, around tubes 1, 2 and 3. The path f is around tube 2. Anamplifier output bridge network Brenders the feedback path F and thecircuit L conjugate, in the manner disclosed in the above mentionedcopending application of H. S. Black. The ratio arms of the bridge areresistances or impedances R0, KRO, KR and R. The voltage fed back bypath I F is applied across resistance 7 in series with the secondarywinding of input transformer T in the input circuit of tube 1.Interstage network 8 couples tubes 1 and 2. Interstage network 9 couplestubes 2' and 3. Only the alternating current circuits of the amplifier Aare shown, the direct current energizing circuits or other circuits forenergizing the amplifier or conditioning it for operation being omittedfor the sake of sim- To accomplish this preplicity as they can readilybe supplied by those skilled in the art. The dotted lines in the figure,indicate that stopping condensers, grid bias volt? age supply sources,etc. are to be added in the cir-. cuit as required.

The negative feedback amplifier A of the closed loop comprising theforwardly transmitting path 'P and the feedback path F, includes aninner and magnitude of the resulting voltage generated in the platecircuit of the last tube, or the voltage of an equivalent fictitiousgenerator in series with the internal plate resistance of the last tube.The amplification of an amplifier, for example, amplifier A, withoutfeedback from the output to the input as for example without thefeedback through path Fv (but with any internal feedback amplifier suchas b producing internal feedback, as through path f), will be designatedas a (and is a complex quantity). The amplification that an internalfeedback amplifier, such for example as amplifierb, would have withoutfeedback as for example without feedback through path i, will bedesignated at (or [La or 0, etc.). Amplification ratio is the absolutevalue of the amplification. Gain is twenty times the logarithm of theamplification ratio.

The (complex) quantity p will be used herein to designate the ratio bywhich a voltage of a For example, p will be used to represent this ratiofor amplifier A, and the corresponding ratio for an internal feedbackamplifier, as for example amplifier b, will be designated bfib, (orMafia, or ie 3c, etc.)

As shown in the above mentioned copending application of H. S. Black,the amplification of a feedback amplifier is and the correspondingchange in amplification caused by the feedback action is With #9., [1b,#0, etc., representing the amplifications or ,us for the individualstages (such as those comprising tubes 1, 2, 3, etc.) of a feedbackamplifier when there is no internal feedback (i. e. when there is nointernal feedback path such as j), the value of p for the amplifier(neglecting interstage losses between plate generators and grids, forsimplicity) is p(I-B fl-b-#c When there is-local feedback for the stagecomprising tube 2 for instance, (as for example through path I), thenthe value of p becomes As explained in the above mentioned copendingapplication .of H. S. Black, the factor can represent either increase ordecrease of amplification, and of gainI For example, an increase of gainmay be obtained when the angle of p is less than 90 and the absolutevalue of 1.5 is two or less depending on the angle; and for any anglewhen IMBI is greater than two and for any value of |m3| when the angleof be is between 90 and 270 the gain is decreased. Since the angle of m3appears in the denominator, the reciprocal of the angle of l bBb appearsin the external m8. When [Lbflb or the internal p49 1, then l bl b isapproximately M51. and the phase shift introduced in the external 43 isopposite in sign and slope to the angle of internal me.

This last property is very important since, as

indicated above, it has not been possible to ob-' tain a. suitable phasecorrective passive network with a negative phase slope, because allnon-dissipative networks give positive slope and dissipative networksgive negative slope over the portion of the frequency range of highloss.

As indicated above'with -bfib 1 and the high pass filter 5 attenuatingwaves in the utilized frequency range and freely transmitting waves ofhigher frequencies and the phase shift of network 6 adjusted to valuessuitable for reducing the minimum departure of the angle of external sfrom zero and multiples of 360 over the frequency range of outer loopgain, the singing margin of the amplifier A can be greatly increased.Figs. 2 to '7 show variousmodifications of parts .of the circuit of Fig.1 including the internal closed feedback loop. To facilitate comparisonofFig. 1 with Figs. 2 to "l, the tube around which the local feedbackpath f is shown in Figs. 2 to 7 is designated 2 in those figures, andthe preceding tube is designated 1 in Figs. 5 to '1. However, it is tobe understood that the amplifier stage shown in Figs. 2 to 4 can be anystage of a multi-stage amplifier with all of its stages included in afeedback loop, such as the amplifier A of Fig. 1, for example; and that,similarly, the

two stages shown in Figs. 5 to 7 can be any two consecutive stages ofsuch an amplifier.. I

Fig. 2 shows the local feedback provided as in Fig. 1, except that inFig. 2 the filter is shown as a band elimination filter, attenuatingwavesof the frequencies'of the utilized frequency range. Fig. 3 showsthe local feedback provided as' in Fig. '1, except that the voltagelocally fed back is applied across a resistance 10 in series in the gridcircuit of tube 2.

Fig. 4 is like Fig. 3 except that the voltage locally fed back isapplied across a resistance or' impedance 11 which, in series-with aresistance or impedance 12 is shunted acrossthe grid sir-- cult of tube2.

Fig. 5 is like Fig. 4 except that-the interstage coupling impedance 8replaces the impedance 12. Fig. 6 shows the local feedback provided asinFig. 1 except that a specific form of high pass filter is shown,consisting of a half section of the constant k type and a half sectionof m derived type, and a specific form of phase adjusting de phase shiftsubstantially independent of fre-' quency, between its input and outputvoltages.

Fig. 7 shows the local feedback provided as 'in Fig. 1 except that thevoltage fed back locally is applied across a resistance or impedance 13in series in the screen grid circuit of tube 1.

The amplifier circuit of Fig.8 is like that of Fig. 1 except that theouter feedback path includes a low pass filter, the local feedback isaround tubes 1 and 2, instead of around tube 2, specific networks areshown in bridge B which corresponds to the bridge B of Fig. 1 and in theblocks corresponding to the blocks shown in Fig. 1, and the tube 3 isshown as a coplanar grid tube of the type described in the abovementioned copending application of H. S. Black. The tubes 1, 2 and 3 canbe of any suitable type. Tube 3 is shown with a filamentary cathodearranged to be heated by current supplied from winding 20, which may be,for example, the secondary winding of a transformer. The cathodes oftubes 1 and 2 may beheated indirectly or in any suitable way.

Tube 1 is coupled to tube 2 through the inter"- stage coupling network 8and a stopping condenser C11 and grid leak resistance R9. Tube 2 iscoupled to tube 3 through the interstage coupling network 9 and stoppingcondenser C13 and grid leak resistance R11. Resistance R5 and con--denser C1 smooth out negative biasing voltage supplied to the controlgrid of tube 3' from battery 25 through the grid leak resistance R11.Resistance Re and condenser Ca smooth out positive biasing voltagesupplied to the space-charge grid of tube 3' from battery 26.

Plate current for tube 1 is supplied from battery-27 through resistanceR19 and other elements of the interstage coupling network 8, thecondenser Cashown in network 8 being a by pass condenser which, inconjunction with resistance R19, smooths out the unidirectional platevoltage for tube 1. The plate current of tube 1 passes throughresistance R1 in the cathode lead.

The resulting voltage drop in the resistance is smoothed out by ashunting or bypass condenser C1 and supplies negative biasing potentialfor the grid of tube 1 through resistance R16 which corresponds toresistance '1 of Fig. L

Plate current for tube 2 is supplied from battery 2'7 through resistanceR20 and other elements of the interstage coupling network 9, thecondenser Ce shown in the network being a by-pass condenser which, inconjunction with resistance R20, smooths out the unidirectional platevoltage for tube 2. The plate current of tube 2 passes throughresistance R3 in the cathode lead. The resulting voltage drop in' theresistance is smoothed out by a shunting or by-pass condenser 04 andsupplies negative biasing potential for the grid of tube 2 through thegrid leak resistance R9.

Screen grid potential for tube 1 is supplied from battery 2'? throughresistance R2, shunted by condenser C2, this resistance and condenser.

smoothing out the potential and adjusting it to the proper value.

Screen grid potential for tube 2 is similarly supplied from battery 2'7through res stance R; and shunting condenser C5.

.Plate current for tube 3' is supplied from battery 27 through chokecoil L9 and elements of bridge B. The coil L9, in conjunction with aby-pass condenser C15 shunted across coil L9 and battery 2'7, smoothsout the unidirectional plate voltage for tube 3'.

Bridge B is an attenuation equalizer, the equalizer 4 supplementing thebridge equalizer so that the two together give the required attenuationequalization for line L as described in the above mentioned copendingapplication of H. S. Black. In the bridge equalizer the elementsdesignated R0, L8 and R12 form the bridge arms corresponding to arms R0,KRD, and KR of the bridge shown in Fig. 1, the reaetance of condenserC15 being negligibly small; and the elements designated L7 and C14 formthe bridge arm corresponding to the arm R of the bridge shown in Fig. 1.

The local or internal feedback path is designated f and includes a highpass filter 5 and all pass network 6 corresponding to the high passfilter 5 and phase adjusting network 6 in Fig. 1 or in Fig. 2. I

The outer feedback path is designated F', and includes a low passfilterwhich has its cut-01f frequency above that of the high pass filter 5 andfunctions as described above to lower the gain around the outer closedfeedback loop of the amplifier at frequencies above those at which thehigh pass filter transmits effectually, for insuring that the outer loopgain is always less than zero when the outer loop phase shift is zero ora multiple of 360 degrees.

Where the utilized frequency range is, for ex ample, from 8 to 100kilocycles, the high pass filter 5 may have its cut-off frequency at 400kilocycles, for instance, and the low pass filter 30 may" have itscut-ofi frequency at 400 kilocycles, for instance, and in the high passfilter 5 the half section of m derived type may give high attentuationat frequencies in the neighborhood of 100 kilocycles and the halfsection of constant It type give high attenuation at lower frequencies,down to 8 kilocycles, for example.

To facilitate practice of the invention, anamber of appropriate circuitconstants for the amplifier of Fig. 8. when it is to amplify waves of afrequency range from 8 to kilocycles are given below. However, suchvalues are merely illustrative, and the invention is not limited hereby.

Tubes 1, 2 and 3 may have amplification constants of 900, 380 and 5.77,respectively, and internal plate resistances of 828,000 ohms, 320,000ohms and 3,900 ohms, respectively.

Values of resistances, inductances and capacities may be as indicated inthe table below, (wherein M.F. signifies microfarads and H signifieshenries) R Ohms C M. F L H.

3 060 3 75 3 0199 4 25500 4 ll 4 1236 5 200 5 01 5 l. 21

8 25000 8 75 8 61. 25X 10- 9 2X10 9 0000955 9 l0. 0

10 51337 10 0000955 10 0124 ll 2X10 ll .07 11 .0036 12 3500 12 000040112 00202 13 2410 13 .07 13 .0561 14 2410 14 000019 14 00975 15 1340 15035 15 .00975 16 3500 16 000288 16 .00255 17 90000 17 001 The curves ofFig. 9 are plots of phase shift versus frequency applying to theamplifier of Fig. 8, over the frequency range from 100-ki1ocycles to1000 kilocycles. The internal 1,8, i.e. the B for the internal closedfeedback loop, is des ignated ltabflab, since the focal feedback isaround tubes 1 and 2. The [Labfiab phase shift, i. e. the angle of[Labfiab or the loop phase shift for the inner loop is shown for thefrequency range just mentioned by curve A. The improvement or 8 withoutthe low pass filter and without the in-' ternal feedback, and curve Dgives the values change in the phase or angle of extern'alj fith at thatthis same 1/3 phase has when the internal I feedback action is takingplace. This curve D shows that more change than necessary was obtained,-and the 1;; phase now crosses through 360. However, the low pass filter30 placed in the outer feedback path changes the total 13 phase to thatshown by curve F. This low pass filter'in addition introduces in Btransmission loss sufficient to insure that the outer loop gain islessthan zero. Singing around the outer loop cannot occur now when thephase does cross back through zero as 13 is now a loss.

As indicated above; since with llnbfiab 1 the gain of the internalfeedback amplifier is substantially pub it follows that if over thefrequency range of interest the angle of were'made equal and opposite tothe angle of the portion of the outer loop that is not common to theinternal loop, then the phase shift around the outer loop would be overthat frequency range. This loop phase shift of 180 would be obtained, inother words, if A, the phase shifttive feedback of waves of saidfrequency but giving the amplifier a tendency to sing at a differentfrequency, and means for reducing the singing tendency comprising afrequency selective negative feedback path for feeding waves back insaid amplifier, said path discriminating in favor of transmissiontherethrough of waves of said different frequency as compared totransmission therethrough of said waves of given frequency.

2. An amplifier, a feedback path for producing in said amplifiernegative feedback of waves of 'thefrequency range of the waves to beamplified,

anda frequency selective negative feedback path I for selectivelyfeeding back negatively in said amplifier waves of frequency exclusiveof the frequency range of the waves to be amplified' 3. An amplifier, a,feedback path for producing in said amplifier negative feedback of wavesof the frequency range of the waves to beamplified, a

to form a closed loop circuit, and means for pro ducing for a portion ofsaid forwardly transmitting path a total resultant phase shift that hasits phase shift-frequency characteristic of negative slope and forms acomponent of the loop phase shift.

5, A wave amplifying system comprising a forwardly transmitting waveamplifying path, a feedback path, a source of waves to be amplified, aload circuit, means connecting said source, said amplifying path, saidload circuit and said feedback path to form a closed loop circuitnormally effecting feedback of waves originating in said' I source thatreduces the gain from source to load below the value it would have withno feedback in the system, and means for producing in said forwardlytransmitting path a phase shift in waves transmitted through that paththat has its phase shift-frequency characteristic of negative slope andforms a component of the loop phase shift.

6. A waveamplifying system. comprising a forwardly transmitting waveamplifying path, a feedback path for producing feedback in said system,a source of waves to be amplified, a load circuit, means connecting saidsource, said amplifying path, said load circuit and said feedback pathto form a closed loop circuit, and means for producing in said forwardlytransmitting path a phase shift in waves transmitted through that path,said phase shift having its phase shiftfrequency characteristic ofnegative slope, and said last mentioned means comprising an amplifieramplifying waves in said forwardly transmitting path and feeding back insaid amplifier waves that reduce the gain of said amplifier below thevalue it would have with no feedback in the amplifier.

7. A wave amplifying system comprising aforwardly transmitting waveamplifying path, a'

feedback path for producing negative feedback in said system, a sourceofwaves to be amplified, a load circuit, means connecting said source,said amplifying path, said load circuit and said feedback path to form aclosed loop circuit having loop gain large compared to unity, and meansfor producing in said forwardly transmitting path a phase shift in wavestransmitted through that path, said phase shift having its phaseshift-frequency characteristic of negative slope, and said lastmentioned means comprising a negative feedback amplifier amplifyingwaves in said forwardly transmitting path.

3. A wave amplifying system comprising a forwardly transmitting waveamplifying path, a feedback path for producing feedback in said system,a source of waves to be amplified, a load circuit, means connecting saidsource, said amplifying path, said load circuit and said feedback pathto form a closed loop circuit, and means for producing in said forwardlytransmitting path a phase shift in waves of selected frequencytransfrequency characteristic .of negative slope and forms a componentof the loop phase shift.

9. A wave amplifying system comprising a forwardly transmitting waveamplifying path, a feedback path for producing negative feedback in saidsystem, a source of waves to be amplified, a load circuit, meansconnecting said source, said amplifying path, said load circuit and saidfeedback path to form a closed loop circuit, and means for producing insaid forwardly transmitting path a phase shift in waves of selectedfrequency transmitted through that path that has its phaseshift-frequency characteristic of negative slope, said last mentionedmeans comprising a negative feedback amplifier amplifying waves insaidforwardly transmitting path and a'high pass filter in said forwardlytransmitting path, having its outoff frequency above the frequency rangeof the waves to be amplified by said system.

10. A wave amplifying system comprising. an amplifier, a negativefeedback path from the output to the input of said amplifier, a secondfeedback path from the output to the input of said amplifier, a highpass filter in the first mentioned feedback path, and an. activetransducer in said other feedback path.

- transducer in said other feedback path, said high pass filter havingits cut-off frequency ofthe order of magnitude of the upper limitingfrequency of the utilized frequency range of waves to be amplified bysaid amplifier, and said low pass filter having a higher cut-offfrequency than said high pass filter. I

12. A wave amplifying system comprising an amplifier, a negativefeedback path therefor, and

a low pass filter in said feedback path, having its cut-off frequencyabove the utilized frequency range of waves to be amplified by saidamplifier, for preventing feedback of waves of frequency above theutilized frequency range that tends to produce singing.

13. A wave amplifying system comprising an,

amplifier, a negative feedback path therefor, an attenuation equalizerin a portion of said path, and means comprising a high pass filterconnected around said portion for reducing the singing tendency of thesystem, said filter having its cut-off frequency at least as high as afrequency approximately that of the upper limit of the utilizedfrequencies of the frequency range to be amplified by said amplifier.

14. A wave amplifying system comprising an amplifier, a negativefeedback path therefor, an attenuation equalizer and a. low pass filterin a portion of said path and a high pass filter connected around saidportion, said high pass filter having its cut-off frequency atleast' ashigh as a frequency approximately that of the upper limit of theutilized frequencies of the frequency range to be ,amplified by saidamplifier, and said low pass filter having a higher cut-off frequencythan said high pass filter.

15. A multi-stage amplifier, a feedback path for all of the stages, andmeans producing frequency selective negative feedback for certain ofthe- 16. A multi-stage amplifier, means feeding back waves from theoutput to the input of said amplifier, and means comprising a high passfilter for producing negative feedback in less than the whole number ofthe stages of waves whose frequency is above the frequency range of thewaves to be amplified in said amplifier.

17. A multiple feedback amplifier comprising three stages of-vacuumtubes connected in tandem, means for producing negative feedback ofwaves from the output of the third stage to the input of the firststage, a local feedback path around less than the whole number of saidstages,

and a high pass filter in said local feedback path.

18. A multi-stage amplifier, means feeding back waves from the output tothe input of said amplifier, and means comprising a high pass filter anda phase shifting network forproducing negative feedback in less than thewhole number of the stages of waves whose frequency is above thefrequency range of the waves to be amplified in said amplifier.

19. A multiple feedback amplifier comprising three stages of vacuumtubes connected in tandem, means for producing negative feedback ofwaves from the third stage to the first stage, a local feedback patharound less than the whole number of said stages, and a high pass filterand phase reversing means in tandem in said local feedback path.

20. A wave amplifying systemcomprising a forwardly transmitting waveamplifying path, a feedback path for producing negative feedback in saidsystem, a source of waves to be amplified, a load circuit, meansconnecting said source, said amplifying path, said load circuit and saidfeedback path to form a closed loop circuit having loop gain largecompared to unity, and means for preventing singing around said loop,said last mentioned means comprising means for producing in saidforwardly transmitting path a phase shift in waves transmittedthroughthat path that has its phase shift-frequency characteristic of negativeslope and increases the minimum departure of the loop phase shift fromzero and multiples of 360 for the frequency range of loop gain.

21. The method of operating a negative feedback amplifier whichcomprises introducing frequency selective gain reduction in the closedfeedback loop and at the same time introducing, in the feedback loop,phase shift with a phase shift-frequency characteristic of negativeslope that increases the minimum departure of the loop phase shift fromzero and multiples of 360 for the frequency range of loop gain.

- GEORGE W. BARNES.

